1. Field of the Invention
The invention relates to a secondary air feeding apparatus for supplying secondary air to a region upstream of an exhaust gas purification unit disposed in an exhaust system of an internal combustion engine. More particularly, the invention relates to a secondary air feeding apparatus whose components can be subjected to a determination on abnormality, and to a method of detecting abnormalities in the components.
2. Description of the Related Art
As an exhaust purification unit for internal combustion engines, there has been known a unit for purifying exhaust gas by reducing the concentrations of CO, HC, and NOx components contained therein by means of a three-way catalyst disposed in an exhaust system. In addition, there has also been known an art for promoting the purification of exhaust gas by forcibly feeding air from an air pump to a secondary air feeding passage having an opening-closing valve connected to an exhaust pipe, thereby increasing the concentration of oxygen, and oxidizing the HC and CO contained in exhaust gas.
If an abnormality occurs in a component such as the air pump or the opening-closing valve in the secondary air feeding apparatus as described above, the efficiency of purifying exhaust gas decreases, which causes a deterioration in exhaust emission properties. Therefore, it is necessary to make a determination on the abnormality at an early stage. In order to detect abnormalities of this kind, therefore, there have been known arts disclosed in Japanese Patent Application Laid-Open No. 9-21312 and Japanese Patent Application Laid-Open No. 9-125945.
According to the former, a pressure sensor is disposed in a secondary air feeding passage between an air pump and an opening-closing valve, and an abnormality in a secondary air feeding apparatus is detected on the basis of a pressure value detected by the pressure sensor. According to the latter, a pressure sensor is disposed in a secondary air feeding passage, and an abnormality in a secondary air feeding apparatus is detected on the basis of a difference between maximum and minimum pressure fluctuation values detected by the pressure sensor.
According to these arts, however, although it is possible to detect an abnormality in the secondary air feeding apparatus itself, it is difficult to precisely determine which component of the apparatus is abnormal. Furthermore, in the case of an operational failure (e.g., the clogging of a line or the like), namely, if pressure values and pressure fluctuation values are normal despite an abnormality in the function of a certain component of the apparatus, the detection of the abnormality is impossible.